It has been proposed to use a laser beam as a spacial reference against which the movement of a portion of a natural or man-made structure can be measured. The laser can be mounted at a reference point to be directed adjacent a structure to be monitored. The portion of the structure monitored carries a target which intercepts the beam to measure the position of the beam relative to the target. In one prior proposal the target normally allows the beam to pass whereby a number of such targets can be mounted at different points of the structure spaced along the beam. One such target employs two orthogonal straight edges individually movable to intercept the beam so that by measuring the positions of the straight edges at the points of interception the beam position is measured in Cartesian coordinates. For a further discussion of this kind of detection system reference may be had to published International Application No. WO85/00222 which was published on Jan. 17th, 1985, that is after the priority date of the present application, and which corresponds to commonly assigned U.S. application Ser. No. 705,469 filed Feb. 21, 1985.
Various other techniques for using laser beams in position measurement have been disclosed in British patent specification Nos. GB 1550934, GB 1511265, GB 1483021, GB 2074808 and GB 2090096A. The present invention is concerned with the detection of a beam by intersection of two target edges with the beam but adopts a different approach to that set out in the abovementioned application WO85/00222.